Solvent activity

Although the phrase 'activity of a solution' usually refers to the activity of the solute in the solution as in the preceding section, we also can refer to the activity of the solvent. Experimentally, solvent activity a1 may be determined as the ratio of the vapour pressure p1 of the solvent in a solution to that of the pure solvent pe, that is ai = fè = Yx pi

where Yi is the solvent activity coefficient and x1 is the mole fraction of solvent.

The relationship between the activities of the components of the solution is expressed by the Gibbs-Duhem equation d(ln a.1) + x2 d(ln a2) = 0 (3.40)

which provides a way of determining the activity of the solute from measurements of vapour pressure.

Water activity and bacterial growth

When the aqueous solution in the environment of a microorganism is concentrated by the addition of solutes such as sucrose, the consequences for microbial growth result mainly from the change in water activity aw. Every microorganism has a limiting aw below which it will not grow. The minimum aw levels for growth of human bacterial pathogens such as streptococci, Klebsiella, Escherichia coli, Corynebacterium, Clostridium perfringens and other clostridia, and Pseudomonas is 0.91.5 Staphylococcus aureus can proliferate at an aw as low as 0.86. Figure 3.2 shows the influence of aw, adjusted by the addition of sucrose, on the growth rate of this microorganism at 35°C and pH 7.0. The control medium, with a water

activity value of a w = 0.993, supported rapid growth of the test organism. Reduction of aw of the medium by addition of sucrose progressively increased generation times and lag periods and lowered the peak cell counts. Complete growth inhibition was achieved at an aw of 0.858 (195 g sucrose per 100 g water) with cell numbers declining slowly throughout the incubation period.

The results reported in this study explain why the old remedy of treating infected wounds with sugar, honey or molasses is successful. When the wound is filled with sugar, the sugar dissolves in the tissue water, creating an environment of low aw, which inhibits bacterial growth. However, the difference in water activity between the tissue and the concentrated sugar solution causes migration of water out of the tissue, hence diluting the sugar and raising aw. Further sugar must then be added to the wound to maintain growth inhibition. Sugar may be applied as a paste with a consistency appropriate to the wound characteristics; thick sugar paste is suitable for cavities with wide openings, a thinner paste with the consistency of thin honey being more suitable for instillation into cavities with small openings.

An in vitro study has been reported6 of the efficacy of such pastes, and also of those prepared using xylose as an alternative to sucrose, in inhibiting the growth of bacteria commonly present in infected wounds. Polyethylene glycol was added to the pastes as a lubricant and hydrogen peroxide was included in the formulation as a preservative. To simulate the dilution that the pastes invariably experience as a result of fluid being drawn into the wound, serum was added to the formulations in varying amounts. Figure 3.3 illustrates the effects of these sucrose pastes on the colony-forming ability of Proteus mirabilis and shows the reduction in efficiency of the pastes as a result of dilution and the consequent increase of their water activity (see Fig. 3.4). It is clear that P. mirabilis was susceptible to the antibacterial activity of the pastes, even when they were diluted by 50%. It was reported that although aw may not be maintained at less